1. Field of the Invention
The present invention relates to a tracking system for a rotary magnetic recording medium, and, more particularly, to a tracking system for tracking a rotary magnetic recording medium such a magnetic disc or a magnetic drum in order to retrieve information therefrom. Still more particularly, the present invention is concerned with a tracking system for reproducing information from concentric tracks formed on a magnetic disc under tracking servo control.
2. Description of the Prior Art
An electronic still camera system has recently been developed which employs in combination an image pickup device such as a solid-state imaging device or an imaging tube and a recording device of the type using, as a recording medium, magnetic discs which are inexpensive and have relatively large storage capacity. A desired object is shot as a still picture purely electronically by the image pickup device to be recorded in a rotating disc by the recording device, the picture being reproduced by means of a television system, a printer or other suitable devices.
The problem encountered with recording media used for such magnetic recording, particularly magnetic discs, is that tracking errors are often invited by their anisotropy, eccentricity or offset, thermal expansion, etc. That is, part of a track adjacent to desired one tends to the be sensed during playback to result in crosstalk.
In order to solve the above problem, it has been proposed to record a tracking signal on a magnetic recording medium together with information under tracking servo control, and reproduce the information with tracking servo effected by use of the tracking signal. However, when it comes to cameras and other small and light recording apparatus, installation of a tracking servo mechanism which requires accurate control is impractical.
Such a situation may be settled by using a guard band recording system or a frequency modulation (FM) azimuth recording system. The system is such that some degree of tracking error is compensated for by preventing a playback head from scanning a nearby track or, if scanned, preventing it from picking up a signal therefrom.
In addition, also used is a so-called "wobbling" system. In accordance with this system, during recording a head is fed at a predetermined track pitch on a track-by-track basis by a stepping motor without tracking servo applied, whereas, during playback, tracking servo is applied by detecting an envelope of a signal retrieved from each track so as to identify an optimum track based on its peak position.
For the convenience of use, it is generally advantageous to construct a playback apparatus for magnetic discs and other rotary magnetic recording media such that a medium is removably loaded and such media recorded with a variety of information are selectively used. Therefore, it cannot be guaranteed that the center of tracks on a medium be accurately aligned with that of a rotary drive shaft of the playback apparatus any time the medium is loaded in the apparatus, that is, the position usually involves some eccentricity. In addition, it cannot be assured that the signals recorded in a medium will be in concentric tracks which are precisely concentric with the center of rotation of the medium.
Where the center of a medium becomes offset to different degrees every time it is loaded in a playback apparatus or where the tracks of the medium per se are eccentric, signals reproduced therefrom will undergo variation in level in unison with the rotation of the medium. Assuming that signals recorded in a medium are field video signals, the level might become high in a former half of a field relative to a latter half, or vice versa, or averaged as a whole with no regard to the contents of the video signals, in dependence on the eccentric condition of the medium.
In the wobbling system, as previously described, tracking control is performed based on the envelope of a recorded signal. Hence, should the envelope level depend on the eccentric position of the medium, such a system would fail to attain accurate tracking control.
Further, the reproduced signal level depends on the orientation of the magnetic material constituting the medium as well as on the manner of contact of the medium with a playback head. Therefore, an optimum track position cannot be detected if the tracking control is effected by such transient changes in envelope level.
Concerning a rotary magnetic recording medium for use with an electronic still camera system, for example, fifty tracks may be formed on a disc whose diameter is as small as about 50 millimeters, at track pitches of the order of 100 microns, i.e., with a track width of the order of 50-60 microns and a guard band width of 50-40 microns. In a playback apparatus, the magnetic disc is driven at a constant rate of 3,600 rpm to reproduce a video signal at a field frequency or a frame frequency. The wobbling system, therefore, requires accurate tracking control the positional accuracy of which is on the order of less than 10 microns.